<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
    .parent{
      width: 400px;
      height: 400px;
      background-color: yellow;
      position: relative;
    }
    .child{
       width: 100px;
       height: 100px;
       background-color: red;
       position: absolute;
       left:50%;
       top:50%;
       margin-left: -50px;
       margin-top: -50px;
    }
    </style>
</head>
<body>
    <div class="parent">
      <div class="child">

      </div>
    </div>
</body>
</html>